This invention relates to a system for drilling underground boreholes. In particular the invention relates to such systems in which a drilling tool is moved through the borehole using a tractor or similar device.
In a conventional drilling setup, a drill bit is mounted on a bottom hole assembly (BHA) that is connected to a drill string made up of tubular members connected in an end-to-end arrangement. The BHA can include measuring instruments, a drilling motor, telemetry systems and generators. Penetration is achieved by rotating the drill bit while applying weight on a bit (WOB). Rotation can be achieved by rotating the drill string at the surface or by use of a drilling motor downhole on which the drill bit is mounted. The drilling motor is typically powered by flow of a drilling fluid through the drill string and into a hydraulic motor in the BHA. The drilling fluid exits through the drill bit and returns to the surface outside the drill string carrying drilled cuttings with it. WOB is applied by the use of heavyweight drill pipe in the drill string above the BHA.
Clearly WOB can only be applied when the heavyweight drill pipe is close to the borehole. When it is desired to drill highly deviated borehole sections (close to horizontal), the heavyweight drill pipe may have to be located some distance from the BHA in order for it to be in a borehole section that is close to vertical.
Another form of drilling uses coiled tubing to connect the BHA to the surface. An example of this is found in Hill D, Nerne E, Ehlig-Economides C and Mollinedo M “Reentry Drilling Gives New Life to Aging Fields” Oilfield Review (Autumn 1996) 4-14 which describes the VIPER Coiled Tubing Drilling System. In this case the coiled tubing is used to push the drilling tool along the well and provide WOB. However, problems can occur as the coiled tubing does not have great strength in compression.
Recently, various proposals have been made for drilling systems conveyed on wireline cable. An example of this is found in PCT/EP04/01167. Clearly a flexible cable cannot be used to provide WOB.
The various problems incurred in obtaining WOB, in conventional, coiled tubing and wireline drilling have led to the development of tractor or thruster devices to provide the necessary WOB. These devices typically lock in the borehole above the drill bit to provide a reaction point and use a drive mechanism to urge the drill bit away from the reaction point and provide WOB.
There have been a number of proposals for tractors and thrusters. Tractors are used to convey borehole tools along the borehole in highly deviated situations. These typically pull the tool(s) on a wireline cable down the well which is then logged back up the well on the wireline cable pulled from the surface. Examples of tractors for such uses can be found in U.S. Pat. No. 5,954,131, U.S. Pat. No. 6,179,055 and U.S. Pat. No. 6,629,568. A tractor for use with coiled tubing or drill pipe is described in U.S. Pat. No. 5,794,703.
Rather than pulling the tool, a thruster pushes a tool forward. Examples of such thrusters can be found in U.S. Pat. No. 6,003,606, U.S. Pat. No. 6,230,813, U.S. Pat. No. 6,629,570 and GB 2 388 132. Thrusters often can be used for pulling as well. The term “tractor” is used in this application to indicate both forms of device. Where a distinction is required, the terms “pulling tractor” and “pushing tractor” are used.
There are various mechanisms used by tractors. In one approach, wheels or chains act on the borehole wall to drive the tractor along. Another approach is a push-pull crawler. In this case, the device locks one end against the borehole wall and extends a free end forward. At the limit of its extent, the free end is then locked and the other end released and retracted to the newly locked end. When fully retracted, the other end is locked and the locked end released and advanced again. This is repeated as required to either push or pull equipment connected to the tractor. This can be used for both pushing and pulling actions.
Drilling using a wireline cable from the bottom-hole drilling assembly (BHA) to the surface offers many benefits in terms of reduction of cost to drill, and reduction of assets and personnel. However, with these comes a reduction in the available power available to drill with. This decrease in power creates the need to optimize the drilling process by applying a lower than conventional force and torque at the bit, and also being able to control the rate of penetration (ROP) or advancement in real time. This control by definition requires drilling assemblies that move slowly. With the limited power available downhole from the wireline cable, the drilling speed is typically limited to a few meters per hour. This drilling tool however also needs to trip in and out of the hole for changing the bit or to modify settings. The wireline cable can be used to trip quickly (up to 3000 m/hr) in the vertical section of the well; but a lot of wells today would have an inclination that can even reach horizontal. Wireline tractors of the type described above are used to quickly run the measurement equipment to bottom and log (even at 3000 m/hr) back up the well. These same tractors though are not optimized for very slow and accurate movement as required for the drilling process.